1. Technical Field
The invention concerns a microelectromechanical structure (MEMS), in particular a sensor which is capable of operating in corrosive gases or liquids with for example a high salt concentration and which in an especial configuration can be a sensor for a microviscosimeter. The invention further concerns a method for the production of such a structure, such a sensor and in particular a microviscosimeter.
2. Discussion of Related Art
DE 100 27 684 B4 discloses a device and a method for viscosity measurement. The operating principle of the viscosimeter described therein will firstly be briefly discussed hereinafter.
Arranged in a measurement zone on a mechanical stable substrate are two or more conductors which are arranged at a small spacing and of which at least one is connected to a controlled current source or to an HF voltage source. A second conductor, here also referred to as the actuator, is held in entirely or portion-wisely cantilevered relationship within the measurement zone. A force required for elastic flexing thereof is generated by a high frequency field between the actuator and a conductor track region which is arranged beneath same and which is also referred to as the base plate, wherein the frequency of the high frequency field is a multiple higher than the natural resonance frequency of the arrangement.
The position of the actuator which is held in cantilever relationship within the measurement zone can be altered, utilizing the elasticity of its holding arrangement or utilizing its own elasticity, by voltage-dependent or current-dependent electrical or magnetic attraction or repulsion forces. Those forces are modified in respect of time by means of the HF voltage sources or current sources.
The capacitance of the arrangement comprising the actuator, the base plate and a medium filling the space between them is a measurement of the elastic flexing of the actuator by the electrical attraction force of the above-mentioned electrical high frequency field. That capacitance reactively influences the oscillator frequency as it is an integrated component of the LC or ring oscillator generating the high frequency field.
By virtue of a change in the attraction or repulsion forces and by virtue of a change in position of the actuator, which is delayed in dependence on the viscosity of the medium to be investigated, it is possible to draw conclusions about the viscosity of the medium to be investigated, by means of an integrated measurement device. The viscosity of a liquid filling the space between the actuator and the base plate can therefore be determined on the basis of the measurable period of time which is required after the oscillator is switched on for predetermined frequency detuning thereof with respect to its starting frequency.
Structural features of the device known from DE 100 27 684 B4 and the process technology for production thereof are briefly described hereinafter.
The actuator is to compromise a very thin elastic and preferably surface-insulated or passivated material. Passivated aluminum is given as an example. A movable conductor loop of aluminum is produced after manufacture of all active and passive components of an integrated circuit of the viscosimeter, by a procedure whereby, prior to the opening of passivation windows and prior to separation of the sensor chips produced on a wafer, an additional photolithographically structured lacquer mask is applied. By means thereof, the part of the uppermost conductor track plane, which is provided for the movable conductor loop, of the integrated circuit is under-etched by a local isotropic insulator etching process and completely separated from the insulating substrate.
The method known from DE 100 27 684 B4 for the production of the actuator of a microviscosimeter using the uppermost conductor track plane of a conventional multi-plane conductor track system suffers from the disadvantage that the proposed actuator structures used of surface-insulated or passivated aluminum Al are not permanently corrosion-resistant in corrosive media such as a blood plasma.
That problem concerns generally MEMS structures which in operation are exposed to corrosive gases or liquids.
The underlying technical object of the present invention is therefore that of providing an MEMS structure or an MEMS component such as for example a sensor, for instance a microviscosimeter, which is particularly resistant in a corrosive medium and which at the same time can be produced with a conventional backend process. From method points of view the object of the invention is to provide a method for the production of such an MEMS component.